An electron beam system with pattern writing capability has come to be widely used in a fabricating process of an ultra-large-scale integrated circuit (ULSI, hereinafter) with the highly accurate and fine patterings, because the minimum feature size of the ULSI exceeds the limitations of optical resolution. Particularly, the electron beam cell projection lithography has attracted engineers in this field, because of its high throughput property. The aforementioned lithography and a mask used in this technology are described in two following literatures. One is "Thermal Characteristics of Si Mask for EB Cell Projection Lithography" by Y. NAKAMURA, H. SATOH and Others, which appeared on Jpn. J. Appl. Phys., Vol. 31 (1992) pp. 4268-4272, Part 1, No. 12B, Dec. 1992. Another one is "Electron-Beam Cell Projection Lithography: A new high-throughput electronbeam direct-writing technology using a specially tailored Si aperture", by Y. NAKAMURA, S. OKAZAKI, and N. SATOH, which appeared on J. Vac. Sci. Technol. B8 (6), Nov/Dec, 1990.
In the electron beam system with pattern writing capability, an electron beam irradiated from an electron gun is shaped into a beam with a rectangular cross-section with desired dimensions by a first beam shaping mask. Thereafter, the electron beam is again shaped into a beam with a cross-section having various miniaturized patterns by a second beam shaping mask. In the beam shaping mask, a Si-substrate is used as a basic component, the top-surface of which is plated by Au. The Si-substrate is used because of its accuracy in the fabrication process, and the Au-film has a role to increase the electron beam intercepting capability.
In order to obtain highly accurate opening patterns, the thickness of the Si-substrate should be decreased, and then the thickness of the Au-film should be increased to secure the electron beam intercepting capability. Subsequently, large thermal stresses arise between the Si-substrate and the Au-film, and the exfoliations of the Au-film may arise. Moreover, since there is no metallic film on the bottom surface of the Si-substrate, electric charges may be stored thereon, and the electron beam may be deformed by electrostatic forces between the electron beam and the electric charges stored on the bottom surface of the Si-substrate.